Application of Gas-Assisted Gravity Drainage (GAGD) Process for Enhancing Recovery from Unconventional Resources





The oil recovery with hydraulic fracturing has played an important role in hydrocarbon production and energy support last decade from unconventional resources. Characteristically, the significant production decline and low recovery factors from these reservoirs triggered the need for new EOR techniques to compensate for the decline and help sustain the production. In this study, an experimental investigation of the Gas-Assisted Gravity Drainage (GAGD) process in the presence of fractures as EOR process was conducted using Nitrogen (N2) and Carbon Dioxide (CO2) in Berea Sandstone (BSS) and Tuscaloosa Marine Shale (TMS).

Core flooding and EOR experiments were used to determine the rock petrophysical properties and investigate the performance of several EOR processes such as continuous gas injection (CGI) and GAGD. The effects of injection direction, reservoir, and operational conditions were extensively studied on BSS cores. The effect of introducing fracture and fracture configuration on EOR was investigated by injecting N2 into BSS core plugs and injecting CO2 into large BSS core samples at optimum operating conditions. The tight core plug TMS was used to study the effects of low permeability (ultra-low permeability) on the EOR process. The mechanisms of oil displacement in porous media are discussed to understand their impact on the EOR process.

The results showed that the N2-GAGD process with fractures can effectively improve the reservoir productivity from unconventional resources by gravity drainage and oil displacement mechanisms. The CO2-GAGD showed promising EOR potential through gravity force, diffusion, evaporation, and lowering oil viscosity, interfacial tension (IFT), and capillary pressure. Introducing fracture in the BSS cores for EOR experiments generally increased the stimulated reservoir volume (SRV). The EOR experiments showed that up to 82% of oil-in-place (OIP) can be recovered using the CO2-GAGD process with fractures from BSS while the oil recovery can reach up to 7.63% OIP from very tight (Shale) TMS core by CO2-GAGD process. The study showed that the GAGD process can be effective in enhancing recovery from fractured reservoirs of low and ultra-low permeabilities found in unconventional shale reservoirs.

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The GAGD process improves oil recovery by accomplishing better sweep efficiency and higher microscopic displacement taking advantage of the natural tendency of fluid gravity segregation to recover the bypassed oil from unswept regions in the reservoir. Besides, the process results in delaying and minimizing water production as the horizontal production well is located at the bottom of the pay zone and above the oil-water contact level. Also, the GAGD process is cost-effective because of the usage of a single well to inject the gases in the reservoir and produce from the reservoir without the need to have multiple wells to achieve various improving oil recovery patterns, especially in the offshore and/or mature fields. GAGD process reported improving the oil recovery to ultimate level compared with the other processes experimentally and simulation modeling in conventional samples (Munawar, Rao, & Khan, 2017), (Dinh et al., 2017), (Saikia, 2016), and (Paidin, 2013). Will the process work in unconventional samples? Is it going to open a new era of improving oil recovery? Are the recoverable reserves going to be double or triple the current numbers?

To address these questions, this study aimed to examine the applicability of the GAGD process in tight and shale oil reservoirs through laboratory experiments designed to understand the mechanism of improving the recovery of unconventional resources. Different schemes were performed experimentally with different operating conditions and injected gases. The gases that have been investigated over the last decade from different studies were CO2, N2, and enriched natural gas (Alfarge et al., 2017).

Table 2. 3. Summary of the Reported Experimental Studies for Gas Injection EOR Techniques in Unconventional Reservoirs



Table 2. 4. Summary of Numerical Simulation Studies for Gas Injection EOR Techniques in Unconventional Reservoirs



Table 2. 5. Summary of Field Applications of Gas Injection Techniques in unconventional reservoirs